Preheat the oven to 400° F.
Butter well a 9-inch pie plate.
Stir the vinegar into the milk to sour it, and let the mixture stand for 10 minutes.
Plump the currants up by covering them with hot water for a few minutes.
Drain.
Mix the dry ingredients together and cut the butter in with a pastry blender until the mixture resembles coarse crumbs.
Stir in the drained currants.
Mix the egg with the milk and pour it all at once into a well in the dry ingredients.
Stir to form a dough.
If it seems too soft to handle, add a bit more flour.
Gently pat the dough into the prepared pan and bake on the top shelf for 20 minutes, or until the center is dry when tested with a cake tester.
Do not overcook! Let the cake stand a few minutes; then use a spatula to loosen it completely from the pan and slide it onto a dish.
Serve warm, cut into wedges, with a nice selection of jams on the side.